Eastern Decidious Forests

1. Locations of the Decidious forests

A little over 1/10 of ( 11%) our continent is covered by an assemblage of decidious broadleaf trees, including the oaks, maple, beech, basswoods, hemlocks, and formely, the chestnuts.

Beyond the northern boundary ( the Canda-US border) the trees are limited by a long enough growing season. The cost of producing new leaves is more than the energy obtained during the short span of summer months. However, some ecologists argue that an additional fact may be limited time for immigration. THese specinores were forced by the last series of glaceiers down south. Tree migration is a slow process, and pollen analysis indicates the trees may still be moving. at a snail's pace.

The western boundary is a result of limited water supplies, especially those of the hot dry summers of the Midwest. Grasses with their deep tap roots are favored in this environment, along with with periodic fires set by lightning flashes igniting dry grass bedding.
Up north the trees can extend a bit further west, as it is the combination of precipitation along with heat that controls transpirational loses ( trees lose 90% of their water here). The cooler temperatures mean less transpiration, while hot southern climes mean higher water loss rates, and the need for at least 35-40" ppt to support tree life relative to the 20-25" required up north.

The southern boundary drops to the coastal plains of southern Georgia, and dips down to the Gulf coast further west. Pines, cypress, evergreen oaks and more subtropical species out compete the deciduous species further south.

2. Deciduous Associations:

Although the eight distinguishable associations all contain some species in common such as oaks, maples or beeches, there are enough differences to clearly distinguish the assemblages.

 

3. The Chestnut Blight and its consequences:

http://www.ppws.vt.edu/griffin/blight.html

The Chestnut oak was once the dominant species in many parts of its range, filling up to near 1/3 of the above canopy. This large tree, up to 33 ft. in circumerface, was critical to the ecosystem. It's large seeds feed many of the larger and smaller herbivores and omnivores including bears, deer, rodents of all types.People used the seeds for their hogs as well. The beautiful and strong wood made up to 1/4 of all the lumber cut in the southern Appalachians. In the early part of the century ( approximately 1904-5), importers of Asian chestnuts unintentionally brought in a fungus which ended up decimating the species in this country. Although the Asian trees were immune, the American species was not. Once it gained entrance to the tree, the fungus would enter the phloem ( thin layer of cells in the bark which transports sugars and other nutrients to all parts of the tree) and plug it up. It didn't take long to kill the individual, and the easy spread of fungal spores by wind and animals allowed the fungus to migrate readily reaching most all trees within a few decades.By 1920 half the trees were killed, and by 1940 the remaining half succumbed- three and a half billion American chestnuts had perished. The spores still remain in the soil and on dead individuals. Sprouts that occasionally come forth from old root systems, not entirely killed off, may make it for 10-20 years before they die back. Not only were the herbivores at a disadvantage with the smaller acorns that took the nuts place, when oaks substituted for the chestnut in the following years, they also had to deal with the fact that oaks don't produce nuts every year like the chestnuts did, but only in 2-5 year cycles dependent on species.

 

 

4.The impact of cycling mast trees: cycles of mammals, trees and Lyme disease:

Oaks as noted above are mast seeders. It is thought they evolved this seeding strategy to reduce herbivory and increase successful germination of new seedlings. By only producing seed periodically and in massive amounts they hope to 'over saturate' organisms who normally feed on their seeds. They remaining 5% of seeds not eaten should form the next generation of trees. Among the mammals most tied to mast seedlings are white-footed mice, chipmunks, and white-tailed deer along with a myriad of insect feeders. These species are themselves fed upon by other creatures which are severely impacted when their populations fall. Eggs and small birds are eaten by chipmunks, and gypsy moth larvae are eaten by the mice. The mice and chipmunks along with birds who eat the acorns are themselves eaten by other predators including raptors, foxes and coyotes. Deer are critical to the survival of the trees. They not only eat the acorn but also seedlings of all species. This limits the understory incredibly, which in turn impacts on birds. Thus:

Good year: Deer move from maple-beech forests to the oak forests ( see assemblages above). These well fed deer are more likely to bear twins instead of the normal single offspring. This surging population of deer picks up ticks, which happily feed on the deer. The following spring there is a massive influx of ticks throughout the forest.

Getting back to the mice: they also increase their offspring number with increased acorns. The following spring just as the larval ticks are at their peak, so is the mouse population. The tick larvae prefer the mice, and transfer the microbe responsible for Lyme's disease into the bodies of the mice. Infected mice in turn infect non-infected ticks when they bite down on the mice.. As mice disperse out after the mast is gone, they carry the disease out to the fields and to human populations which now become infected.

So how does this relate to the gypsy moths? Another introduced species, ( back in 1800's), their larvae feed on oak leaves along with other species of trees. When massive cycles of gypsy moths arrive and defoliate the trees, the oaks suffer if not die out right. Those that survive will not have the energy to produce acorns at least for a year or more.

Now back to the mice.. when their populations are high, they keep the moth populations down significantly and can't rebound till the mice are gone. The mice are so efficient they can eat up to 90% of the larvae - but when the mice are low, only 50% of the larvae are eaten.. Perhaps one 'good' effect of the gypsy tree kills are that they increase the light on the canopy floor allowing seedlings and seed to germinate and 'rejuvenate' the forest. Thus each of the players acts on each other, helping to regulate their prey's population size but also in the process be regulated themselves.

 

5. Why do deciduous trees drop their leaves ( when it costs so much to produce new ones each year?)

Unlike the deciduous species, gymnosperm wood is made up primarily of tracheids. These are thin, elongate cells which carry water from the root to the shoot and needles. The long cells are tapered at the ends, and in order for water to move from one cell to the next, it must pass through tiny holes or pit pairs adjacent on two cells. In hardwoods, instead of tracheids, the xylem is made up of tube like structures, with one open ended cell sitting above the cell below.. Rather like short straws, attached one to another at the ends. This maximizes the speed with which the water can flow upward, unlike the gymnosperms in which water must maneuver through small adjacent pores. However, this faster flow costs the trees in the winter. With first frost, the probability that water moving through the deciduous xylem stacks becomes less likely.
Warm water can hold less gas, cold water more gas ( i.e.. trout can only live in colder well oxygenated waters). When the temperatures drop, as with the colder nights of fall, the water can absorb more gases. However with warming during the day, gases will come out of solution. If the difference in temperature is great enough, air or gas bubbles may form in the xylem tubes and break the water column. Once broken, it is difficult to repair, and if water is not brought laterally to reconstitute the column, the leaves above will not be able to get the water they need. Additionally, the large % of water in the leaf tissue itself, would cause leaves to freeze and wilt with expansion and breakage of cell vacuoles.

Below note the gymnosperm wood on the left and the deciduous wood on the right.

cross-section gymnosperm

Cross-section hardwood

Tangential section gymnosperm

Tangential section hardwood

 

6. Fungi: who runs the show? mycorrhizea and fungal nets

From Kendrick (1999)

As a rule, mycorrhizal infection enhances plant growth, by increasing nutrient uptake in up to 3 ways: i)   by increasing the surface area of absorption within the soil, ii)  by mobilizing sparingly available nutrient sources from unavailable compounds, and iii) by excreting chelating compounds or ectoenzymes (Marschner & Dell, 1994). The fungal sheath can act as a storage site for nutrients, and allowing the fungi to continue to provide nutrients to the plant when soil concentrations decrease. In return for receiving extra nutrients, mycorrhizal plants loose between 10% and 20% of the photosynthates they produce, which go to the formation, maintenance and functioning of the mycorrhizal fungi and their associated structures. . Direct uptake of nutrients by plant roots can often result in zones of nutrient deficiency around the root system.  Subsequent absorption of nutrients is then limited entirely by the rate at which nutrients move through the soil. Hyphae can grow past these depletion zones and therefore maintain the rate of absorption.  Maintaining and extending hyphae is a more economical way of sustaining contact with nutrients than by continually extending roots The rate at which nutrients move through the soil can be crucial to their availability.  Phosphorus, in the form of phosphate (PO4-), has very low mobility in soil, and therefore tends to be the limiting nutrient in most ecosystems.  In return for receiving extra nutrients, mycorrhizal plants loose between 10% and 20% of the photosynthates they produce, which go to the formation, maintenance and functioning of the mycorrhizal fungi and their associated structures. Ectomycorrhizas can also confer pathogen resistance to their plant partner.

 

7. What do the shape of leaves tell you? ( partial Tree identification: see: http://wwwfac.mcdaniel.edu/Biology/appliedbotany/treeidlab/treeidset.html)

Leaf shape is a well designed features of plants.

Heat gain in leaves depends on: leaf angle to sun, leaf surface reflectivity, leaf size, leaf shape.
Heat loss from leaves depends on
* leaf angle to the wind, which affects evaporative and convective cooling rate. In some species the petiole will follow the sun to maximize incident radiation.
* Leaf size and leaf shape (edges lose heat faster than interior areas, so heat exchange rate is dependent on the ratio of leaf perimeter to leaf area) Smaller leaves will do better in colder areas, larger leaves in warmer areas.
* Leaf thickness (surfaces lose heat faster than interior areas, so heat exchange rate is also dependent on the ratio of leaf surface area to leaf volume). Leaves or needles in colder areas will maximize volume over surface area, but compensate for loss of leaf surface but bundling needles closer together.

It has been hypothesized that the serrated margin gives a greater circumference to the leaf, allowing a greater density of pores near the edge of the leaf and hence allowing a greater rate of transpiration. Another theory is that the serrated margin affects the turbulence around the leaf and hence heat transfer from the leaf

Besides leaf temperature, light conductance is very important. In a tall tree, if the leaves were broad and intercepted all the light, leaves below would be in the shade. The cost of supporting leaves that don't produce enough energy to sustain themselves would be a drain on the tree. Once way around this is to create lobed leaves, which permit leaves to pass some of the sunlight to its neighbors below.

In subcanopy trees that barley receive enough light to maintain themselves, the leaves are quite broad without lobes, as the light they would pass would not be sufficient to sustain another layer.

In tropical trees, the leaf edges are quite smooth and end in a drip tip. Here the idea is to funnel the water off the leaf as quickly as possible to minimize nutrient loss through leaching of the leaf and to dry the leaf before various moss, algae and bacteria take residence.

Thus leaf shape and size are important considerations in the survival of trees.

 

 

	Leaves:shade versus sun leaves.
		Shade leaves are: * thinner and weakly lobed * large surface area * less support tissue fewer stomata
		*Sun leaves are: * more xerophytic in nature, * possess thicker cuticles, * more and longer palisade cells * well developed vascular tissues.
	Which of these 2 is the shade leaf? how can you tell?

8. How old are these trees? Tree coring and history

A quick look at the longevities below give some indication of the wide distribution of ages that these species may obtain. Although the ages listed below are more likely maximums for the species, with most trees surviving less that these ages due to competition and environmental insults ( storms, droughts, insect plagues), the top trees in the canopy could readily approach these age classes.

Why the variance? a sort of rough rule of thumb is the faster growing, the softer the wood, the shorter lived the species. Slow growing oaks often live the longest relative to fast growing cherries and birches. In early successional species, fast growing short lived species start the process ( i.e. locusts, cherries), but are gradually overtaken by slower, longer lived species which rise over them in the canopy.

Eastern Hemlock Tsuga canadensis	300-1000
Balsam Fir Abies balsamea	80 years, fungus rots stem from inside at about 40 years old Associates: Spruces and hardwoods
Red Spruce Picea rubens	300-400
White Spruce Picea glauca	200
Black Spruce Picea mariana	250
White Pine Pinus strobus	200+
Red Pine Pinus resinosa	350
HARDWOODS
Sugar Maple Acer saccharum	300-400
American Beech Fagus grandifolia	300
Yellow Birch Betula alleghaniensis	150-300
White Ash Fraxinus americana	150+
Red Maple Acer rubrum	70-100-(150)
Northern Red Oak Quercus rubra	200-300
Black Cherry Prunus serotina	150-200
Paper Birch Betula papyrifera	80
Pin Cherry Prunus pensylvanica	50
White oak	500-600

9. ACID Rain and it impact on the Decidius Forests up North:

Acid rain has been implicated in contributing to forest degradation, especially in high-elevation spruce trees that populate the ridges of the Appalachian Mountains from Maine to Georgia, as well as maples that inhabit the NE forests. Acidic deposition seems to impair the trees' growth in several ways; for example, acidic cloud water at high elevations may increase the susceptibility of the red spruce to winter injury.

In New England - White & Green Mts. 1/2 of Red Spruce have died.

Why so much damage?There is good reason to believe that long-term changes in the chemistry of some sensitive soils may have already occurred as a result of acid rain. As acid rain moves through the soils, it can strip away vital plant nutrients through chemical reactions, thus posing a potential threat to future forest productivity.
"
short article: Trees Need Calcium, Too... Soil Calcium Depletion Linked to Acid Rain and Forest Growth in the Eastern United States

• Calcium levels in forest soils have decreased at locations in 10 states in the eastern United States, according to a new report by the U.S. Geological Survey, Department of the Interior. This trend is a cause for concern because calcium is necessary for neutralizing acid rain and is an essential nutrient for tree growth.

  Sugar maple and red spruce trees, in particular, are showing reduced resistance to such stresses as insect defoliation and low winter temperatures. These stresses have been linked to decreases in calcium available to the trees. Although the specific relationships among acid rain, the availability of calcium in soil, and forest growth remain uncertain, ongoing research by the USGS, in collaboration with the U.S. Forest Service, the University of Illinois, the Institute of Ecosystem Studies, and Syracuse University, has provided new information on the causes, magnitude and effects of calcium depletion in the soils of the Eastern U.S.

  "USGS research has identified a new mechanism by which acid rain decreases the availability of calcium in the soil," said Gregory Lawrence, USGS research scientist, Troy, N.Y., and co-author of the report. "Through this mechanism, acid rain releases aluminum from the underlying mineral soil layer, which is followed by the upward transport of the aluminum into the forest floor (the nutrient rich organic soil layer where root activity is greatest) by root uptake and water movement. The result is that aluminum replaces calcium and the trees have a harder time trying to get the needed calcium from the soil layer."

  "When we took a regional look at the forest-soil chemistry of red spruce trees, we could see a trend toward soil calcium depletion in the Northeast," said Lawrence. "Three independent studies of soil calcium show that recent measurements fall well below those measured 50 years ago."

  Thomas Huntington, USGS research scientist, Atlanta, Ga., and co-author of the report, said, "USGS research in the Southeast shows that depletion of calcium in soils is common in that region as well, and that growth of forests may be limited by calcium availability. This situation may be worsened by forest harvesting in areas currently low in soil calcium." Timber harvesting can contribute to the problem because the calcium removed with the trees would otherwise be recycled within the ecosystem as leaves and branches decompose.

Depletion of calcium in forest soils may also explain why, despite decreases in acid rain over the past three decades, that stream-water chemistry has shown minimal recovery at many locations in the Northeast, such as the Neversink watershed in the Catskill Mountains of New York, where long-term trends in calcium concentrations and acid-neutralizing capacity (an integrated measure of acidity resistance) in stream water have shown significant decreases over the past 15 years.